[Flightgear-flightmodel] Ground Effect in YASim

[_ __ <rit...@gm...>]

YASim currently calculates ground effect as a linear lift increase starting
from one whole wingspan above the ground. In a previous email
<https://sourceforge.net/p/flightgear/mailman/flightgear-devel/thread/CAF9yP-fcYZKR%3DF%2B_j%3DwUeRL4A9s6DuGW1Ndxz8uzYRGNYo_%2BrA%40mail.gmail.com/#msg58834280>
I mentioned that this should be a non-linear increase, and custom compiled
my own version with a low effort fix.

I am now back with a more detailed proposal, following equation 11 from this
study
<https://www.researchgate.net/publication/269047238_Lifting-Line_Predictions_for_Induced_Drag_and_Lift_in_Ground_Effect>,
using the following code (the bolded stuff is the new stuff):

from /flightgear-release-2024.1/src/FDM/YASim/Airplane.cpp line 599:

    if(_wing) {
        // Ground effect
        // If a double tapered wing is modelled with wing and mstab, wing
must
        // be outboard to get correct wingspan.
        float pos[3];
        float gespan = 0;
        gespan = _wing->getSpan();


*float gefactor = pow( gespan, 2) / _wing->getArea();        gefactor = 288
/ pow( gefactor, 0.882);*

        _wing->getBase(pos);
        if(!isVersionOrNewer( Version::YASIM_VERSION_2017_2 )) {
          //old code
          //float span = _length * Math::cos(_sweep) * Math::cos(_dihedral);
          //span = 2*(span + Math::abs(_base[2]));
          gespan -= 2*pos[1]; // cut away base (y-distance)
          gespan += 2*Math::abs(pos[2]); // add (wrong) z-distance
        }
        if (baseN != 0)
          baseN->getChild("wing", 0)->getNode("gnd-eff-span",
true)->setFloatValue(gespan);
        // where does the hard coded factor 0.15 come from?


*      // changing it to a function of aspect ratio
_model.setGroundEffect(pos, gespan, gefactor);*
    }

>From /flightgear-release-2024.1/src/FDM/YASim/Model.cpp line 354:

    // Account for ground effect by multiplying the vertical force
    // component by an amount linear with the fraction of the wingspan
    // above the ground.

    *// changed to non-linear function*
    if ((_wingSpan != 0) && (_groundEffect != 0 ))
    {
      // distance between ground and wing ref. point
      float dist = ground[3] - Math::dot3(ground, _geRefPoint);
      float fz = 0;
      float geForce[3] = {0, 0, 0};








*float h_over_b = dist / _wingSpan;            fz = Math::dot3(faero,
ground);      fz *= pow( h_over_b, 0.787);      fz *= pow( 2.718, -9.14 *
pow( h_over_b, 0.327) );      fz *= _groundEffect;      Math::mul3(fz,
ground, geForce);      _body.addForce(geForce);*

Regards,
Joel